This invention relates to a screen display device for displaying characters and patterns on a TV screen or other device.
Recently, as TVs and VCRs have become more varied, more capable and more user friendly, it has become possible to use characters and patterns to display such information as the current channel and the operating mode on the screen.
FIG. 3 is a block diagram of the design of a conventional screen display device of this type. In FIG. 3, an H counter 1 counts the synchronization signal from the synchronization signal generator circuit (not shown), compares it with the data from the data bus, and detects the character display position. FIG. 3 also depicts screen display oscillator circuit 2; V counter 3 which counts the output clock of screen display oscillator circuit 2, compares it with data from the data bus, and detects the character display position; display position control circuit 4 which uses the outputs of H counter 1 and V counter 3 to control the position of the character display; timing generator 5 which controls the timing of the address counters etc. described below; address counter 6 which outputs the address of the display RAM (display memory) described below; display RAM 7 which stores the addresses of each display color data and character code that is the address of the character ROM (character memory) described below; character ROM 8 stores font data for display characters; shift register 9 converts the output signal of character display ROM 8 into serial data; color signal selection circuit 10 determines the mixing of color signals according to display color data from display RAM 7; bordering signal output circuit 11 outputs bordering data according to the display character font data stored in character ROM 8; shift register 12 converts bordering pattern data from bordering signal output circuit 11 into serial data; and image signal output circuit 13 which outputs a video signal, synchronized with the timing signal from timing generator 5, which comprises, depending upon the output of color signals based on the color signal from color signal selection circuit 10 and font data from character ROM 8 (outputs of shift registers 9 and 12), a red signal from the R (red) terminal, a green signal from the G (green) terminal, a blue signal from the B (blue) terminal, an intensity signal from the I (intensity) terminal, and a blanking signal which erases the background of the character from the OUT terminal and makes the border of a character.
Next the operation of the conventional device is explained. Display position control circuit 4 compares character display position data from the data bus connected to the CPU (not shown) with the values of H counter 1 and V counter 3 to determine the position at which a character is displayed on a screen. Next, according to the outputs of timing generator 5 and display control circuit 4, the contents of address counter 6 are incremented by one for each character and then character type data (address of character ROM 8) and character color data (mixing of the output color signal) which was entered previously in display ROM 7 is read. Color signal selection circuit 10 decodes the read color display data and the output color signal and the selection data are inputted to video signal output circuit 13.
If a border is to be added to the displayed characters, once the shift register 12 has converted the bordering data generated by bordering signal output circuit 11 using character font data from character ROM 8 into serial data b, the serial data b is synchronized with a timing signal from timing generator 5 and a video signal is outputted from the OUT terminal of video signal output circuit 13. In this way a border is created for characters which are displayed on television screens and other types of displays.
By repeating these operations, screen display data is gradually read out from display RAM 7 and character ROM 8 so that a video signal is outputted from video signal output circuit 13.
The conventional screen display device as described above can assign only one type of color signal and blanking signal to each character. Therefore, difficulties arise if two or more screens are displayed on a conventional screen display using a different character output method for each screen, as for example in the case of picture recording, one screen is the original video image, while information needed for the picture recording is displayed on the other screen.